Semiconductor devices comprising a semiconductor element mounted on a substrate are well-known. The wire-chip bonding and the flip-chip bonding are effected in mounting the semiconductor element on the substrate. Flip-chip bonding is widely used, because it is a trend to decrease the pitch between the electrodes. To effect the flip-chip bonding, solder balls are provided on the electrodes of a semiconductor element and are joined to the electrodes of the substrate while heating the semiconductor element and the substrate. As a result, the semiconductor element is electrically and mechanically bonded to the substrate.
Recently, there is a tendency to decrease the thickness of the substrate. As the thickness decreases, the substrate tends to be deformed upon receiving thermal load and, hence, to be warped or undulated. Therefore, it has been attempted to provide an annular stiffener on the surface of the substrate surrounding the semiconductor element to prevent the deformation of the substrate relying upon the stiffness of the stiffener. A semiconductor device provided with an annular stiffener is disclosed in, for example, JP-A-9-260527.
According to this prior art, the annular stiffener is made of a material having a thermal expansion coefficient greater than the thermal expansion coefficient of the substrate. When the substrate is subjected to a high-temperature state, the annular stiffener expands more than the substrate; i.e., the annular stiffener pulls the substrate outward so as to maintain it flat.
However, the substrate is generally returned back to the normal-temperature state from the high-temperature state. Then, the annular stiffener contracts more greatly than the substrate, whereby the substrate contracts. Therefore, the substrate does not assume a completely flat state and adversely affects the subsequent processing steps such as joining using the BGA balls.